284 Absteacts OF the Makerstoun Observations, 1843. 



Mean Meteorological Seasons. 



Spring, The maximum between 9** p.m. and 5'^ a.m. The minimum at S^ 30™ p.m. 

 „ ( The maximum before 5*^ a.m. A secondary minimum about 2*^ p.m. 



' \ A secondary maximum about S^ 20™ p.m. The minimum about 5*^ 



Winter, 



P.M. 



Autumn, The maximum between 9^ p.m. and 6^ a.m. The minimum about I'' 40™ p.m. 

 The maximum after 9** p.m. The minimum before 5^ a.m. 

 ^ A maximum at 9^^ 40™ a.m. A minimum at 2^ 20™ p.m. 



Astronomical Seasons. 



„ . r The maximum after 9'^ p.m. A minimum before 5'' a.m. 



oprmg, I ^ maximum at 8^ 10™ a.m. The minimum at 2^ 40™ p.m. 

 Summer, The maximum before 5*^ a.m. The minimum at 4** 40™ p.m. 

 Autumn, The maximum before 5"^ a.m. The minimum about 4*^ 20™ p.m. 

 ™.. , r The maximum after 9'' p.m. The minimum before 5*^ a.m. 



' \ A maximum at 9^ 40™ a.m. A minimum at 1^ 40™ p.m. 



The diurnal variation for the year 1843 gives — 



The maximum between 9 p.m. and 5 a.m. 

 The minimum at S^ 20™ p.m. 



The division, then, of the total pressure of the atmosphere into two parts, namely, the dry air and aqueous 

 vapour, indicates only a single maximum and minimum in the diurnal curve for the year (as has been shewn 

 by M. Dove and Colonel Sabine). The minimum pressure of dry air takes place nearly two hours after the 

 maximum temperature. We have still the double maximum and minimum in the Winter months. ^Tiy are 

 both exhibited in the pressure of the dry air in Winter ? We have seen that the diurnal range of the aqueous 

 vapour follows, to some extent, the diurnal range of temperature, and that it is greater in Summer than in 

 Winter. We have also seen that, for the year 1843, the diurnal variation of the total atmosplieric pressure 

 has a greater range in Winter than in Summer. It seems curious that the diurnal range of total pressure 

 should diminish when the ranges of its two components increase, and that, when the diurnal ranges of the two 

 components diminish, the diurnal range of the compound pressure should increase. There is no difficulty in 

 seeing, even if the theory be at fault, or if the pressure of aqueous vapour be inaccurately determined, that 

 the comparatively great diurnal range of aqueous vapour pressure for the summer months, and for the year, 

 will swamp the smaller range of the total pressure, and produce a curve for the dry air, with a single maximum 

 and minimum, inverse to that for the pressure of aqueous vapour. The fact that, when the diurnal range of 

 the aqueous vapour pressvire is least, namely, in Winter, the diurnal range of the total pressure is greatest, 

 and the double maximum and minimum most distinctly marked both for the assumed dry air and total pres- 

 sures, leave this mode of resolution with its original difficulties. 



It was pointed out, Table XI., that a secondary maximum and minimum of the pressure of aqueous 

 vapour occurred in several months about 1*^, 3^', and 5*^ p.m. No such periods were observed for the humidity 

 or total atmospheric pressure. Of course, then, in assuming the total pressure to be composed of the pressures 

 of dry air and of aqueous vapour (as calculated), we may expect to find the dry air accommodating itself to the 

 calculated vapour pressure. Accordingly, as has been noticed above, secondary minima and maxima of dry 

 air occur at the same times as the secondary maxima and minima of aqueous vapour. This does not seem 

 very probable. 



